The present invention relates to a solid state imaging device and a method of manufacturing the same.
The solid state imaging device employed in the prior art will be explained with reference to FIG. 14 to FIG. 19 hereunder.
FIG. 14 is a plan view showing a solid state imaging device in the prior art. In FIG. 14, 101 denotes an n-type semiconductor substrate, and a p-well (not shown) made of p-type semiconductor layer is formed on a surface layer portion. Then, a plurality of photoelectric conversion devices 103 that are aligned at a predetermined distance in the column direction (vertical direction in FIG. 14) and the row direction (horizontal direction in FIG. 14) are formed on the p-well.
Charges accumulated in the photoelectric conversion devices 103 are read out to vertical charge transfer paths 105 after a predetermined time has elapsed. Then, a predetermined drive pulse is applied to vertical charge transfer electrodes (not shown) that are formed over the vertical charge transfer paths 105, and thus the charges are transferred toward the downstream (toward lower side in FIG. 14) of the vertical charge transfer paths 105. The vertical charge transfer paths 105 are formed by forming an n-type semiconductor layer on the p-well, and extend substantially in the column direction to pass through between the photoelectric conversion devices 103.
The charges that are transferred in this manner reach eventually a horizontal charge transfer path 106. The charges that reach the horizontal charge transfer path 106 comes up to an output circuit 107 by applying a predetermined drive pulse to a horizontal charge transfer electrode (not shown) that is formed over the horizontal charge transfer path 106. The transferred charges are converted into a predetermined signal at the output circuit 107 and then output to the outside.
FIG. 15 is an enlarged view showing a pertinent portion of the solid state imaging device shown in FIG. 14 in the prior art. In FIG. 15, 104 denotes a read gate portion. The charges accumulated in the photoelectric conversion devices 103 are transferred to the vertical charge transfer paths 105 through the read gate portion 104 by applying a field shift pulse to the vertical charge transfer electrodes (not shown) formed over the read gate portions 104. Also, 102 denotes a device isolation region which is formed by doping high concentration impurity (B (boron)) into the surface layer portion of the p-well. The device isolation regions 102 extend substantially in the column direction to pass through between the photoelectric conversion devices 103.
FIG. 16 is a view showing the solid state imaging device in the prior art to which the vertical charge transfer electrodes 108 that are omitted in FIG. 15 are provided. As shown in FIG. 16, the vertical charge transfer electrode 108 has the well-known one pixel-two electrode structure. The interlace reading is performed by applying the well-known four phase drive pulse to the vertical charge transfer electrodes 108.
FIG. 17 is a view showing an example of such four phase drive pulse. In FIG. 17, VH is a voltage applied when the charges accumulated in respective photoelectric conversion devices 103 are shifted to the vertical charge transfer paths 105 through the read gate portion 104 (when field shifting), and has a voltage of 15 V, for example. Also, VM and VL are voltages applied when the charges in the vertical charge transfer paths 105 are transferred to the downstream, and have a voltage of 0 V and xe2x88x928 V respectively, for example.
In the solid state imaging device shown in FIG. 15 in the prior art, the vertical charge transfer path 105 in a region A and a region B have following features. That is, in the region A, the device isolation region 102 is formed only on one side of the vertical charge transfer path 105. In other words, only one side of the vertical charge transfer path 105 in the region A is defined by contacting to the device isolation region 102.
In contrast, in the region B, the device isolation region 102 is formed on both sides of the vertical charge transfer path 105. In other words, both sides of the vertical charge transfer path 105 in the region B are defined by contacting to the device isolation region 102.
Therefore, an amount of the impurity (B (boron)) that diffuses from the device isolation region 102 into the vertical charge transfer path 105 in the region B is increased rather than that in the region A. The reason for this can be given as follows. That is, in the region A, since the device isolation region 102 is formed only on one side of the vertical charge transfer path 105, diffusion of the impurity (B (boron)) occurs only from this device isolation region 102 formed on one side. On the contrary, in the region B, since the device isolation region 102 is formed on both sides of the vertical charge transfer path 105, diffusion of the impurity (B (boron)) occurs from both sides of the vertical charge transfer path 105. Accordingly, an amount of the impurity (B (boron)) that diffuses into the vertical charge transfer path 105 in the region B is increased rather than that in the region A.
In general, if the impurity is diffused into the vertical charge transfer path 105, a height of potential of the vertical charge transfer path 105 is increased by the so-called narrow channel effect. Then, if an amount of diffused impurity is different between the region A and the region B, the height of potential of the vertical charge transfer path 105 in the region B becomes higher than that in the region A.
This point will be explained with reference to FIGS. 18A and 18B and FIG. 19 hereunder. FIG. 18A is a view showing a sectional shape of the solid state imaging device in the prior art, taken along a C-D line in FIG. 15, and a schematic behavior of potential in the sectional shape. FIG. 18B is a view showing a sectional shape of the solid state imaging device in the prior art, taken along an E-F line in FIG. 15, and a schematic behavior of potential in the sectional shape. In FIGS. 18A and 18B, the vertical charge transfer electrodes 108 (108a, 108b) that are omitted in FIG. 15 are also shown. Also, as is evident from FIG. 15, a C-D sectional shape is one sectional shape in the region A, and an E-F sectional shape is one sectional shape in the region B.
Also, a curve indicated by a solid line in FIGS. 18A and 18B shows potential when the voltage applied to the vertical charge transfer electrodes (108a, 108b) is at a low level (VL), while a curve indicated by a broken line shows potential when the applied voltage is at a middle level (VM).
As shown in FIG. 18A, in the C-D sectional shape in FIG. 15, when the voltage applied to the vertical charge transfer electrode 108a is at a low level (VL), a height of potential of the vertical charge transfer path 105 is HL1. Then, when the applied voltage is at a middle level (VM), the height of potential of the vertical charge transfer path 105 is HM1.
In contrast, as shown in FIG. 18B, in the E-F sectional shape in FIG. 15, when the voltage applied to the vertical charge transfer electrode 108b is at a low level (VL), a height of potential of the vertical charge transfer path 105 is HL2. Then, when the applied voltage is at a middle level (VM), the height of potential of the vertical charge transfer path 105 is HM2.
Because of the aforementioned difference in the amount of diffused impurity (B (boron)), HL2 is higher than HL1 (HL2 greater than HL1), and HM2 is higher than HM1 (HM2 greater than HM1).
FIG. 19 is a view showing a sectional shape of the solid state imaging device in the prior art, taken along a G-H line in FIG. 16, and a schematic behavior of potential in the sectional shape. A curve indicated by a solid line in FIG. 19 shows behavior of potential in the G-H sectional shape when the voltage applied to the vertical charge transfer electrode 108a is at a low level (VL) and the voltage applied to the vertical charge transfer electrode 108b is at a middle level (VM). Also, a curve indicated by a broken line in FIG. 19 shows behavior of the potential when the voltage applied to both the vertical charge transfer electrodes 108a and 108b is at a middle level (VM).
As described above, when the voltage applied to the vertical charge transfer electrode 108a is at a low level (VL), HL2 is higher than HL1. Therefore, as shown in FIG. 19, when the voltage applied to the vertical charge transfer electrode 108a is at a low level (VL) and the voltage applied to the vertical charge transfer electrode 108b is at a middle level (VM), a convex portion whose height is HL2xe2x88x92HL1 is formed in the potential of the vertical charge transfer path 105 in the neighborhood of a coherent region between the region A and the region B.
If such convex portion is formed, motion of the charges that are to be transferred from the left to the right in FIG. 19 is disturbed by the convex portion and cannot be transferred as desired, and thus a vertical transfer efficiency of the solid state imaging device becomes worse. If the vertical transfer efficiency is degraded in this manner, the characteristics of the overall solid state imaging device also become worse.
It is an object of the present invention to provide a solid state imaging device capable of preventing deterioration of a vertical transfer efficiency due to a narrow channel effect and a method of manufacturing the same.
According to the solid state imaging device of the present invention, a plurality of photoelectric conversion devices are formed on the semiconductor substrate at predetermined intervals in a column direction and a row direction respectively. A plurality of device isolation regions is formed on the semiconductor substrate to extend substantially in a column direction. Each of a plurality of device isolation regions extends also in a zigzag direction to pass through between the photoelectric conversion devices. The device isolation regions contain first conductivity type semiconductor layers.
Also, vertical charge transfer paths are formed between the adjacent device isolation regions on the semiconductor substrate, and contain first conductivity type semiconductor layers. The vertical charge transfer paths are formed to extend in a zigzag and substantially in column direction to pass through between the photoelectric conversion devices
The vertical charge transfer paths have a portion whose both sides are defined by the device isolation regions and a portion whose only one side is defined by the device isolation regions. Then, in the present invention, an impurity concentration of the device isolation regions defining the both sides is lower than that of the device isolation regions defining the only one side, so as to reduce the narrow channel effect in the portions of the vertical charge transfer paths whose both sides are defined by the device isolation region.
Accordingly, the situation where the convex portion of potential is formed in the vertical charge transfer paths by the narrow channel effect cannot arise. Thus, the charges in the vertical charge transfer paths can be transferred smoothly and also degradation of the vertical charge transfer efficiency can be prevented.
According to another solid state imaging device of the present invention, the width of the device isolation region that defines both sides of the path is narrower than that of the device isolation region that defines only one side of the path. According to this structure, since an amount of the impurity being diffused from the portion of the device isolation region, which defines both sides of the path, into the vertical charge transfer path can be reduced, the narrow channel effect can be reduced.
Also, according to still another solid state imaging device of the present invention, the vertical charge transfer path has at least (i) first transfer portion whose only one side is defined by contacting the device isolation regions and (ii) second transfer portions whose both sides are defined by contacting the device isolation regions. The first transfer portion and the second transfer portion are connected in the coherent regions located in a downward of the first transfer portions. Then, the vertical charge transfer electrodes are arranged in such a way that one edge portion of the vertical charge transfer path is positioned in vicinity of the coherent regions.
If the vertical charge transfer electrodes are arranged in this manner, the convex portion of potential does not exist in the vertical charge transfer paths since the fringing electric field generated by one edge portion of the vertical charge transfer electrode is canceled by the convex portion of potential in the vertical charge transfer paths in vicinity of the coherent regions by the narrow channel effect. Thus, the transfer of the charges in the vertical charge transfer path can be made smoothly, and also the degradation of the vertical charge transfer efficiency can be prevented.
Also, according to the solid state imaging device manufacturing method of the present invention, there is contained the step of preparing a substrate on which a plurality of photoelectric conversion devices are formed at predetermined intervals in a column direction and a row direction respectively. Then, after this step, there is executed the step of forming a vertical charge transfer path that extend in a zigzag and substantially in a column direction to pass through between the photoelectric conversion devices, by doping a first impurity into portions of the substrate that correspond to the vertical charge transfer path. Then, after this step, there is executed, the step of forming device isolation region that defines one side of the vertical charge transfer path, by doping a second impurity into portions of the vertical charge transfer path, that correspond to the one side.
According to this method, since the first impurity and the second impurity are compensated with each other, the substantial impurity concentration in the device isolation region that define only one side of the path can be reduced.